1. Field of Invention
The invention is related to semiconductor device structures and fabrication methods, and, more particularly, to high power devices and their fabrication.
2. Discussion of Related Art
The reverse-biased pn junctions in high-voltage semiconductor devices can exhibit 15 relatively low breakdown voltages due to electric field concentration at the termination of the pn junction. To mitigate this problem, some high-voltage devices include junction termination extensions (JTEs) that reduce the magnitude of field concentration. A JTE can extend laterally from a junction to spread a field over a larger area, and thus increase a voltage at which avalanche breakdown may occur.
The laterally varying charge within a JTE can improve the blocking performance of the semiconductor junction. The improved junctions are less prone to high voltage breakdown and current leakage when the junction is reverse-biased.
A JTE can be fabricated though use of ion implantation of dopant into areas of a device adjacent to a pn junction termination. In materials where diffusion of dopants is practical (such as silicon), a uniform implant into an area next to a junction termination can be used along with a drive-in step at elevated temperature to distribute the dopant away from the junction termination.
Silicon carbide (SiC) is recognized as well suited to high voltage, high temperature, and high power applications. With the development of power devices and power electronics, silicon-based power devices are reaching their material limits. Wide band gap materials, such as SiC, are therefore viewed as an alternative for power device and electronics applications, particularly for extreme environments. SiC can be superior to silicon because of its wide band-gap, high critical electric field, and superior thermal conductivity.
Since SiC has very low dopant diffusivity, implanted dopant generally cannot be redistributed by annealing to modify its concentration distribution. To form a JTE in SiC, different implant doses into multiple spaced zones can be used to create a non-uniform implant profile extending away from a junction termination. Multiple implantation steps can, however, increase process cycle time and complexity as well as fabrication cost.
Use of an implant mask having a tapered thickness can provide a JTE doping profile with a graded junction depth and graded concentration extending away from the junction termination. Formation of a tapered mask, however, is difficult to control.
As an alternative, portions of the SiC can removed via etching to create a JTE. This approach also entails difficult process control, and over etching of the SiC is difficult to remedy. Moreover, though commonly used, Reactive Ion Etching (RIE) can leave residual material damage resulting in increased leakage currents and diminished blocking performance.